Underpotential adlayer structures

Using the electrochemical AFM, Manne et al. (1991a) studied the problem of the underpotential deposition of Cu on Au in more detail. Some of their results are shown in Fig. 16.15. Several unexpected phenomena were discovered. First, the structure of the Cu adlayer on Au(lll) depends dramatically on the nature of the electrolyte. With CUSO4 solution, the observation of Magnussen et al. (1990) was completely confirmed. However, in Cu(C104)2 solution, the Cu adlayer becomes close packed with a 30 10° rotation relative to the top-layer Au structure. Second, in some cases, a monolayer step of the Cu adlayer with a well-defined border is observed. At... 

Atomic structures of several adlayers of Cd deposited underpotentially on Au(lll) surface in H2SO4 solution have been visualized applying in situ STM [418]. Three ordered adlattices have been observed, all of which had a long-range linear morphology and were rotated by 30° with respect to the substrate lattice directions. The same system has been studied later... 

Consequently, the formation of the Pb adlayer in this underpotential range can be considered as an 1/2 localized adsorption on a square lattice. In this case each adatom in the compact monolayer covers effectively two adsorption sites. Thus, domains with an Ag(100)-c(2 x 2) Pb structure located on different substrate sublattices Oike white and black fields of a chessboard) separated by mismatch boundaries are obtained as shown, for example, by Monte Carlo simulation (cf. Section 8.4) of 1/2 adsorption on a square lattice [3.214], The fit of experimental coverage data of the first Pb adsorption step on Ag(lOO) (cf. Fig. 3.9) by Monte Carlo simulation is illustrated in Fig. 3.30. From this fit, a lateral attraction energy between the Pb adatoms of V Pbads-Pbads 2.5 X 10 J (corresponding to 1.5 x 10 J mole ) can be estimated [3.184, 3.190, 3.191, 3.214]. Preferential Me adsorption on surface heterogeneities like monatomic steps was disregarded in the fit procedure. 

The adsorption of anions such as halides, cyanide, and sulfate/bisulfate on electrode surfaces is currently one of the most important subjects in electrochemistry [1 - 3]. It is well known that various electrochemical surface processes such as underpotential deposition of hydrogen and metal ions are strongly affected by co-adsorbed anions. Particularly, structures of the iodine adlayers on Pt, Rh, Pd, Au, and Ag surfaces have... 

The second example involves the surface chemistry of the compound semiconductor CdSe synthesized epitaxially on Au(100) by underpotential deposition (UPD). By analogy with the gas-phase epitaxial deposition procedure, this UPD-based method has been dubbed electrochemical atomic layer epitaxy (ECALE) [6]. Unique information on the interfacial structure of the first adlayer of Se electrodeposited was revealed by STM experiments. 

Surface X-ray scattering (SXS) The first SXS study of an underpotentially deposited metal monolayer was reported more than ten years ago. In a recent review [132] it is demonstrated that this method is well suited for the study of the structure of metals, halides, and metal-halide adlayers on single-crystal electrodes. As another example, the study of the distribution of water at Ag(lll) surface can be mentioned [133]. 

A main field of activities is focused on structure and reactivity in two-dimensional adlayers at electrode surfaces. Significant new insights were obtained into the specific adsorption and phase formation of anions and organic monolayers as well as into the underpotential deposition of metal ions on foreign substrates. The in situ application of structure-sensitive methods with an atomic-scale spatial resolution, and a time resolution up to a few microseconds revealed rich, potential-dependent phase behavior. Randomly disordered phases, lattice gas adsorption, commensurate and incommensurate (compressible and/or rotated) stmctures were observed. Attempts have been developed, often on the basis of concepts of 2D surface physics, to rationalize the observed phase changes and transitions by competing lateral adsorbate-adsorbate and adsorbate-substrate interactions. 

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